Glaucoma is a general term applicable to diseases of the eye in which the intraocular pressure in the eye increases to a level which can cause damage to the structures within the eye. If the pressure increases suddenly as in acute angle closure glaucoma, surgical intervention is immediately required to reduce the intraocular pressure and prevent permanent damage to the eye structure. When the pressure increase is gradual as in open angle and neovascular glaucoma, one mode of treatment for reducing the pressure involves treatment with medication, for example, by a topical application to the eye and/or by oral administration. In more severe cases when the pressure resists control by medication, more aggressive intervention involves the installation of a shunt valve which connects the anterior chamber to the subconjunctival space. This promotes drainage or the fluid from the anterior chamber so that the pressure in the eye can be relieved.
In one embodiment of a shunt valve, referred to as a standard length "Krupin" glaucoma valve implant, the implant comprises an open plastic tube which is placed into the anterior chamber, an elastic tube fixed to the plastic tube and plastic side arms extending laterally of the plastic tube for fixation of the implant. The elastic tube has a closed distal end with vertical and horizontal slits forming unidirectional and pressure sensitive valve openings. The distant end of the elastic tube remains outside the eye. In order to install the implant it is necessary to make an external conjunctival incision and to form scleral and conjunctival flaps. The implant is placed in the bed of the scleral flap. The plastic tube must be cut to length during surgery so that the open end of the plastic tube will not contact the iris or cornea. The implant is sutured in place using the side arms as anchor points in order to prevent anterior--posterior movement. The scleral flap is closed by suturing and the conjunctiva is also closed by suturing.
In another embodiment, known as the long "Krupin" glaucoma implant, the valve end of the elastic tube is first sutured within the groove of an elastic band and the band is sutured against the sclera. A notch is formed in the elastic band through which the elastic tube extends for connection to the plastic tube. The elastic tube is sutured to the sclera. The elastic band with the elastic tube therein extends beneath both horizontal rectus muscles. As in the first described embodiment, the plastic tube is placed in the scleral bed and penetrates into the anterior chamber. Sutures are necessary to fixate the plastic tube to the sclera and to close the scleral flap and the conjunctiva. The end of the elastic tube with the valve slits remains within the subconjunctival space.
In another embodiment, known as a Molteno implant, an open silicone rubber tube extends 2 to 4 mm into the anterior chamber and the tube is attached to and opens onto the upper surface of a thin acrylic episcleral plate 13 mm in diameter. The edge of the plate has a thickened rim that is perforated to permit attachment of the tube. The plate is sutured to the sclera. As in the other embodiments, scleral and conjunctival flaps are formed. The tube is sutured to the sclera at the posterior edge of the scleral flaps.
All of the above embodiments and variations thereof have numerous disadvantages and relatively low success rates. They all involve substantial trauma to the eye and require great surgical skill. The eye is exposed to long periods of surgery and considerable manipulation of tissue. The implants themselves are complex and require careful installation to prevent leakage, damage to the lens or iris, dissection of muscle tissue as well as blockage of flow through the tube passages. Frequently, it is necessary to perform corrective surgery to overcome complications due to the implant surgery.